1. Field of the Invention
The present invention relates to an optical disk apparatus and an optical disk, and, specifically, to an optical disk apparatus carrying out at least information reproduction from among information recording, information reproduction and information deletion on an optical disk having a plurality of recording layers, and an optical disk on which at least information reproduction from among information recording, information reproduction and information deletion is carried out with the use of the optical disk apparatus.
2. Description of the Related Art
Recently, along with progress of digital technology and improvement of data compression technology, an optical disk such as a DVD (digital versatile disk) takes an attention as an information recording medium storing information such as music, movie, photograph, computer software or such (which may be referred to as ‘contents’, hereinafter). Then, thanks to price reduction thereof, an optical disk apparatus used to handle the optical disk as an information recording medium has wide spread.
As an amount of the contents tends to increase year after year, increase in a recording capacity of the optical disk is demanded. In order to increase the recording capacity of the optical disk, one method is to increase a recording density and another method is to increase the number of recording layers included in the optical disk.
As to the increase in the recording density of the optical disk, study has been proceeded with for shortening a wavelength of a light beam applied to the optical disk as well as reducing a diameter of a beam spot (spot diameter) formed on the recording layer by means of an objective lens. Then, standardization for a next generation DVD configured to have a light beam of approximately 400 nm applied thereto (which may be called a ‘blue DVD’), which is further shorter wavelength than a wavelength of a light beam of approximately 660 nm which a current DVD is configured to have applied thereto has been proceeded with eagerly.
On the other hand, as to the increase of the number of recording layers in a single optical disk, development of an optical disk having a plurality of recording layers (referred to as ‘multi-layer disk’ hereinafter), and development of an optical disk apparatus configured to access the multi-layer disk have been proceeded with actively (see Japanese Laid-open Patent Applications Nos. 8-96406 and 9-54981, for example). As to the multi-layer disk, spacing between recording layers (inter-layer distance) is an essential matter. When the inter-layer distance is short, so-called crosstalk may occur, in which not only a signal from an intended recording layer but also a signal from another recording layer may be included in a returning light beam from the multi-layer disk at a high level. In such a case, as a result, an S/N of a reproduced signal may degrade. On the other hand, when the inter-layer distance is large, a signal from the intended recording layer may degrade due to influence of spherical aberration. For example, in a one-side two-layer DVD-ROM, the distance between two recording layers is defined as being 55±15 μm. According to Japanese Laid-open Patent Application No. 8-96406, the upper limit of the distance between recording layers is defined as being 7.5 n3λ/{(n2−1)NA4}, where n denotes an equivalent refractive index between the recording layers; λ denotes a wavelength of an applied light beam; and NA denotes a numerical aperture of an objective lens.
Then, by combining the shortening of a wavelength of a light beam and increase in the number of recording layers, remarkable increase in the recording capacity may be expected. However, when the wavelength of the light beam is shortened, the upper limit of the distance between two recording layers is reduced further than that of the above-mentioned one-side two-layer DVD-ROM (70 μm). As a result, it is difficult to use a conventional manufacturing apparatus for manufacturing such a type of an optical disk, and the cost of the optical disk may increase much.